1. Field of the Invention
The present invention relates generally to the interworking of Combined Circuit Switched (CS) call and Internet Protocol Multimedia Subsystem (IMS) session (hereinafter referred to as “CSI”) service. In particular, the present invention relates to a communication method and apparatus between a terminal supporting both a CS call and an IMS session (hereinafter referred to as a “CSI terminal”) and a terminal using an IMS session for real-time voice or video call (hereinafter referred to as an “IMS terminal”).
2. Description of the Related Art
A CS scheme, used for the typical voice service and real-time service, refers to a scheme for forming a CS-based fixed call route between one user and another user. The IMS increases transmission efficiency and guarantees stability using a Packet Switched (PS)-based unfixed call route based on Internet Protocol (IP), and is therefore, suitable for a data service, a messaging service, and a file transfer service. The IMS can support not only the simple point-to-point call, but also a connection in which a plurality of users participate. In the CS scheme, transmission of message and user traffic is achieved through a CS call, and in the IMS, transmission of message and user traffic is achieved through an IMS session.
A service for enabling a terminal to use both the CS call and the IMS session is called ‘CSI service’, and the CSI service uses the CS call especially for voice transmission, and at the same time, uses the IMS session for an improved PS service, for example, the messaging or file transfer service. A terminal (or User Equipment (UE)) supporting the CSI service is called a ‘CSI terminal (or CSI UE)’. The CSI UE uses the CS call for a real-time voice call, and at the same time, can use the IMS session for a non-real-time PS service such as the messaging service. The CSI service, though it considers a voice call as a service using the CS call, can enable the user to use the CS call not only for the voice call, but also for the video call when a CS network supports the video call.
Hereinafter, background of embodiments of the present invention will be described with reference to a voice call service as a typical example of the service using the CS call, and to a messaging service using Message Session Relay Protocol (MSRP) as a typical example of the PS service using IMS, for convenience of description.
FIG. 1 is a diagram schematically illustrating the architecture of a mobile communication network supporting CSI service. Herein, the drawing shows the architecture of a Universal Mobile Telecommunication Service (UMTS) mobile communication network based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS).
Referring to FIG. 1, the mobile communication network supporting CSI service includes a Radio Access Network (RAN) 104 and a Core Network (CN) 106. The RAN 104 enables a User Equipment (UE) 102 to access the CN 106. A structure of the RAN 104 is well known to those skilled in the art according to the wireless access technology used, so a detailed description thereof will be omitted herein.
The CN 106 includes a CS domain 114, a PS domain 108, and an IMS domain 122. The CS domain 114 is comprised of entities supporting the CS call, and the entities typically include a Mobile Switching Center (MSC) 120 and Media Gateways (MGWs) 116 and 118. The MSC 120 processes the signals originating/terminating in a mobile base station in the RAN 104, and takes charge of a central control function of controlling the mobile base station such that it can efficiently operate. The MGWs 116 and 118 are gateways supporting bearer traffics and control traffics.
The PS domain 108 supports a packet-based service, and includes such nodes as a Serving GPRS Support Node (SGSN) 110 and a Gateway GPRS Support Node (GGSN) 112. The SGSN 110 manages information on the location of each UE, and performs security-related functions and access control. The GGSN 112 is an element of a GPRS network serving as a wireless gateway between the SGSN 110 and an external network 130 such as a Packet Data Network (PDN), and the UE 102 can access the external network 130 via the GGSN 112.
The IMS domain 122 is comprised of entities supporting an IMS session. The entities include a Serving-Call Session Control Function (S-CSCF) 124, an Application Server (AS) 126, and a Media Gateway Control Function/Media Gateway (MGCF/MGW) 128. The S-CSCF 124 serves to set up an IMS session and handle a Session Initiation Protocol (SIP) message delivered from the UE 102. To provide a service at the request of the UE 102, the S-CSCF 124 delivers an SIP message to other entities in the IMS domain 122, completing the session. The AS 126 is an entity for providing various applications to the user, like E-mail or Push To Talk over Cellular (PoC). The MGCF/MGW 128 is a network entity for enabling communication between an IMS user and a CS user, and includes an MGCF, which is a gateway for managing signaling translation for interworking between an IMS network and a legacy network, for example, Public Switched Telephone Network (PSTN) or the CS domain 114, and an IMS-MGW for media conversion. Although not illustrated, the IMS domain 122 can further include a Breakout Gateway Control Function (BGCF), which is a network entity for selecting the optimal MGCF which is a contact connected to the CS domain 114 taking into account routing optimization for a CS termination call. The BGCF can be included in the MGCF/MGW 128, or can be separately provided, and is a network entity for selecting the MGCF which is the contact connected to the CS network.
Aside from the foregoing entities, several other entities may exist in each domain, and the entities of each domain may participate for the CSI service.
FIG. 2 is a diagram illustrating connection architecture for control signaling between CSI UEs according to the typical CSI service.
Referring to FIG. 2, a CSI UE#1 204 and a CSI UE#2 214 are illustrated accessing a network#1 202 and a network#2 212, respectively. The network#1 202 is comprised of a CS domain#1 208, a PS domain#1 206, and an IMS domain#1 210. The network#2 212 is comprised of a CS domain#2 218, a PS domain#2 216, and an IMS domain#2 220.
A control signal 232 of a CS call for voice communication between the CSI UE#1 204 and the CSI UE#2 214 is delivered via the CS domain#1 208 and the CS domain#2 218. In addition, control messages 230 of an IMS session for the other multimedia service, except for the voice communication, are delivered via the PS domain#1 206, the IMS domain#1 210, the IMS domain#2 220, and the PS domain#2 216. The IMS session control can be performed with an SIP protocol. SIP is a protocol developed as an alternative to H.323 by Internet Engineering Task Force (IETF) Multiparty Multimedia Session Control (MMUSIC) Working Group. SIP has a platform for setting up voice and multimedia calls via an IP network.
In this way, if two UEs in communication are both CSI UEs, control flow is achieved for each individual domain. However, if one UE does not support CSI service, for example, uses only the IMS session, it cannot follow the above control flow.
Accordingly, a need exists for a system and method for providing communication between a UE supporting the CSI service and a UE not supporting CSI service or a UE using only the IMS session.